A virtual memory technique has been employed in computing machine systems in recent years in order to deal with an increase of the information volume to be processed. Virtual memory is a technique in which data is transferred between a secondary memory unit and a primary memory unit as appropriate to allow an application program to operate as if there is a memory (a virtual memory) with a large capacity and a high access speed. To attain the technique, the primary memory unit (a main memory) and the secondary memory unit (a virtual memory space) are provided. The primary memory unit includes a volatile memory, a nonvolatile memory and the like each having a high memory access speed and a relatively small capacity, and the secondary memory unit includes a hard disk and the like having a lower memory access speed but a larger capacity than the primary memory unit. A paging technique based on pages is one of techniques concerning replacement control between the virtual memory space and a real memory space as well as translation control from a virtual address to a real address. In address translation by paging, a virtual page on a virtual memory is linked to a physical page on a physical memory. Japanese Patent Application Publication No. 2008-158773 discloses a virtual memory management apparatus to perform address translation by paging.
However, according to the virtual memory management apparatus, virtual pages include a clean page for which the same contents are stored in the primary memory unit and the secondary memory unit, and a dirty page for which the memory contents rewritten in the primary memory unit is not reflected in the secondary memory unit. In the virtual memory, data of a clean page will not be lost, but data of a dirty page will be lost in case of power outage. Once the data of the dirty page is lost, the data cannot be recovered even when a page table is reconstructed by rebooting a computing machine.